A polarizing plate has been usually used in a structure, in which a protective film is stacked on both surfaces of a polarizer formed of a polyvinyl alcohol (hereinafter, referred to as ‘PVA’)-based resin dyed with a dichroic dye or iodine. This is because there is a problem in that when the protective film does not exist, durability and optical properties of the polarizer significantly deteriorate due to the vulnerable dimensional stability, and water resistance thereof also becomes notably vulnerable. In this case, a triacetyl cellulose (TAC)-based film has been frequently used as the protective film because the film is excellent in optical transparency or moisture permeability.
Meanwhile, as a liquid crystal display device has recently been developed into a mobile device such as a laptop personal computer, a cellular phone and a car navigation system, it is required that a polarizing plate which constitutes a liquid crystal display device is thin and light weight. However, in a polarizing plate in which a TAC film and the like are laminated as the protective film as described above, it is difficult to maintain a thickness of the protective film at 20 μm or less from the viewpoint of handling property or durability performance during the operation, so that there is a limitation for the liquid crystal device to be thin and light weight.
In order to solve the aforementioned problems, a technology has been suggested, in which a transparent thin film layer is formed by forming a protective film on only one surface of a polarizer, and coating an active energy ray curable composition on the other surface thereof. Meanwhile, the active energy ray curable composition which has been suggested may be classified into a radical curable composition and a cation curable composition according to the curing method. In this case, when a transparent thin film layer is formed using a cation curable composition, the transparent thin film layer is advantageous in that excellent adhesion with the polarizer is achieved, but disadvantageous in that a curling problem of a polarizing plate generated from a slow curing rate, a low degree of curing, a dark reaction and the like occur.
In order to solve such problems, a technology has been suggested, in which a transparent thin film layer is formed by using a radical curable composition which includes an acrylic or acrylamide-based compound as a main component, but in this case, there is a problem in that a polarizing plate, which is manufactured by using the composition, does not have good heat resistance because adhesion deteriorates under a high humidity environment, and a transparent thin film layer to be formed has a low glass transition temperature.
Therefore, there is a need for a new polarizing plate, which is excellent in adhesion between a polarizer and a protective layer, maintains adhesion at a good level even under a high humidity environment, and is excellent in heat resistance while having a thin protective layer which may be formed by curing radicals.